Hydride resonance

Table 3.18 shows how the position of the low-frequency hydride resonance is affected by the trans-ligand, while study of a series of complexes trans-[Pt(PMe2Ph)2(Me)L]+ and neutral trans-Pt(PMe2Ph)2(Me)X shows the trans-influence of the ligand on 2/(195Pt- H) with better donors tending to reduce the value of J (Table 3.20) [152],... 

A ruthenium porphyrin hydride complex was lirst prepared by protonation of the dianion, [Ru(TTP) in THF using benzoic acid or water as the proton source. The diamagnetic complex, formulated as the anionic Ru(If) hydride Ru(TTP)(H )(THF)l , showed by H NMR spectroscopy that the two faces of the porphyrin were not equivalent, and the hydride resonance appeared dramatically shifted upheld to —57.04 ppm. The hydride ligand in the osmium analogue resonates at —66.06 ppm. Reaction of [Ru(TTP)(H)(THF)j with excess benzoic-acid led to loss of the hydride ligand and formation of Ru(TTP)(THF)2. 

The structure of [TpBut]ZnH has been determined by x-ray diffraction, although the hydride ligand was not located (Fig. 38). However, definitive evidence for the presence of the hydride ligand is provided by NMR and IR spectroscopies. Specifically, the hydride resonance is observed at 8 5.36 ppm in the H NMR spectrum, and p(Zn-H) is observed as a strong absorption at 1770 cm-1 in the IR spectrum, which shifts to 1270 cm 1 (vhIpd = 1.39) upon deuterium substitution (Fig. 39). 

The nitrosyls RuH(NO)(PR3)3 are 5-coordinate with trigonal bipyramidal structures and linear Ru-N-O geometries the hydride and nitrosyl ligands occupy the apical positions (for RuH(NO)(PPh3)3, z/(Ru-H) 1970 cm-1, i/(N—O) 1640 cm-1 H NMR, 8 = +6.6 ppm for the hydride resonance). The high-field NMR line is a quartet showing coupling with three equivalent phosphines, which would not be possible in a square pyramidal... 

The binuclear Rh-complex in Figure 11.2 contains two hydride resonances at -11.18 and -14.75 ppm [21]. The former has two smaller phosphorus couplings... 

A variety of examples of 2D-NMR experiments is provided in reference [21]. The structure elucidation of the di-rhodium compound shown in Figure 11.3 was mostly carried out in this way. For example, 2D 11 l-31P heteronuclear multiple quantum correlation (HMQC) experiments were used to show that two rhodium-coupled hydride resonances are connected to a single type of 31P nucleus. 

If the H-NMR spectrum is recorded with complete 31P decoupling, both hydride resonance at -9.4 ppm and -17.6 ppm collapse into a doublet of antiphase doublets. The remaining 13-Hz and 22-Hz couplings correspond to JHRh and JHrk,... 

Furthermore, the intensities of the polarized hydride resonances increase with temperature. Since these intensities correlate with the rate of the oxidative addi-... 

The IR spectrum of 6 shows a vpt jj vibration at 2033 cm indicative of a terminal rather than bridging hydride, and a weak stretch at 2258 cm. The hydride resonance in the H... 

Figure 3. The 100-MHz 1H NMR spectra at 40°C of a CDtCN solution of cis-Os(CO)kHt (0.85 M) and K[Os(CO)kH] (0.34 M), with (MJ and without (M0) irradiation at the hydride resonance of K[Os(CO)kH (indicated by the arrow in lower spectrum). Ratio Mx M0 = 0.7. Chemical shifts are illustrated in 8. The signal at 81.93 is due to residual solvent protons. Hexamethyldisiloxane (0.20 M), resonance 80.05, is an internal intensity standard.

RhH(Cp )(Binap)](SbF5), a presumed intermediate in the hydrosilylation of phenyl acetylene [40], shows the hydride resonance at d -10.39. Hydride resonances in Ru(ii) phosphine complexes are often found in the same region [41-43]. 

Changing the gas flow to 5 bar of CO/Hj (1/1) resulted in reformation of the hydride complex 18b (spectrum d. Figure 6.12). The H NMR spectrum showed the reappearance of the hydride resonance at -10.6 ppm together with an aldehyde resonance at 9.3 ppm. Repeating this sequential procedure with this NMR sample... 

Figure 4.18 Temperature dependence of the chemical shift of the hydride resonance of (triphos)Ru(CO)H2 in CD2CI2 (squares) and in the presence of a twofold excess of (CF3)2CH0H (circles). (Reproduced with permission from ref. 28.)...

Since the nature of the hydride chemical shifts, particularly in transition metal hydride complexes, is not simple [32], there is no reliable correlation between Sh and the enthalpy of dihydrogen bonding. Nevertheless, the chemical shifts of hydride resonances and their changes with temperature and the concentration of proton-donor components, for example, can be used to obtain the energy parameters for dihydrogen bonding in solution. As earlier, the enthalpy (A/f°) and entropy (AS°) values can be obtained on the basis of equilibrium constants determined at different temperatures. Let us demonstrate some examples of such determinations. 

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